The present invention relates to a circular accelerator for accelerating charged particles such as electrons, protons and the like, and more specifically to a circular accelerator which is capable of injecting a large quantity of charged particles in a short period of time, and storing a large current therein.
In a prior art circular accelerator, a beam having a large quantity of charged particles is injected into a beam duct which defines an orbital trajectory, to orbit therein. The injected beam emits a light of emission during its orbit, and its beam size tends to reduce, a phenomenon which is called radiation damping. Because of a varied degree of this radiation damping, depending on the energies of charged particles, prior art circular accelerators are classified roughly into two types in terms of the energy at injection: those in which the energy of charged particles is less than 100 Mev, and those in which the energy of charged particles is 100 Mev or more. In the case of former (more particularly when the energy is less than 50 Mev), because of the relatively small radiation damping effect, it takes a great time for a beam size to be reduced. (For example, at 15 MeV the radiation damping time constant .iota. required for the beam size to become 1/e is approximately 750 s; while at 50 MeV it is 20 s.) Therefore, in most prior art circular accelerators, a beam is injected once for a given period of time, and is accelerated thereafter as described in the Monthly Physics "Accelerator Physics (3)" pp 4.-11 (1985.1).
In another prior art circular accelerator, after a beam is injected, it is accelerated to increase its energy, thereby enhancing the radiation damping effect, and the beam energy is then decreased to an initial condition once again to repeat injection, thereby permitting multiple injections of charged particles, and enabling a large current to be stored therein as set forth in the Ishikawajima Harima Technical Report Vol. 30 No. 5 pp 321-323 (1990.9).
It should be noted that the latter device has a large energy even at the time of injection. Therefore, a substantial radiation damping effect may be expected from the start. Hence, in a third approach after waiting for a reduction in a beam size for a given period of time following the injection, another injection is repeated, so as to obtain a large current.
An important disadvantage of the first of the above prior art devices is that a large current cannot be obtained because as a practical matter only one injection is permitted due to the prolonged radiation damping time required. According to the second prior art device referred to above, the number of repetitions of the injection is at most three times, and a large current is also unlikely to be obtained. In addition, it takes a long time (several minutes) for injection because of the prolonged acceleration and deceleration required. In particular when a superconductivity electromagnet is utilized as a bending electromagnet, its acceleration and deceleration will be prolonged even more. The third prior art approach mentioned above, raises problems that it cannot be applied when injection energy is low, and that, in order to increase the injection energy, the use of a linear accelerator having a length of approximately 20 m, or a microtron is required. The former requires a large-scale system, and the latter requires eventually a lot of time because of the limited number of charged particles permitted to be injected per injection.